Centraliser

ABSTRACT

A centraliser primarily for use when casing an oil or gas well, the centraliser having low frictions coatings or sliders on the outer surface thereof.

This Application is the U.S. National Phase Application of PCTInternational Application No. PCT/GB01/00174 filed Jan. 18, 2001.

This invention relates to a centraliser and relates more particularlybut not exclusively to a casing centraliser for facilitating thecementing of casing in a well.

DESCRIPTION OF THE RELATED ART

When a well has been drilled for the eventual production ofhydrocarbons, one of the procedures commonly employed in readying thewell for production comprises placing a hollow tubular casing in thewell, and filling the space between the exterior of the casing and thewell bore with cement, principally as a sealant and also as a mechanicalsupport. It is desirable that the casing is centralised in the well borewhen cemented, and proposals have been made for providing the casing(prior to cementing) with externally mounted centralisers to hold thecasing away from the well bore and towards the centre of the bore.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided acentraliser having a body with a bore therethrough for receiving atubular to be centralised, the body having one or more low frictionsliders on the outer surface thereof.

The invention also provides a centraliser having a body with a boretherethrough for receiving a tubular to be centralised, the body havinga low friction coating.

The centraliser typically has blades on its outer surface to bearagainst the wall of a borehole and the slider may comprise the whole orpart of a blade. The blades are typically arranged in a peripheral arraycircumferentially distributed around said body to define a flow pathbetween each circumferentially adjacent pair of blades. Each flow pathtypically provides a fluid flow path between longitudinally oppositeends of said centraliser, and each blade typically has a radially outeredge providing a well bore-contacting surface. In a simple embodiment ofthe centraliser the radially outer edge of at least one blade has a lowfriction coating, strip or block applied thereto by any convenientmeans.

The inner surface can also have sliders, coatings or strips appliedthereto in order to reduce frictional resistance to rotation of thecentraliser on the tubular.

The centraliser is preferably a casing centraliser.

The invention also provides a centraliser assembly comprising acentraliser and tubular casing extending longitudinally through the boreof the body. The bore is typically a clearance fit around the tubularcasing to be centralised by the centraliser.

The centraliser is preferably free of any means tightly gripping acasing when said centraliser is installed on it, so that the centraliserand casing can rotate relative to one another.

The centraliser body can be made wholly of partially of metals such asZinc, Steel or Aluminium, or can be of composite materials such asfibreglass, or any other suitable material. We have successfully usedthe “ZA” range of zinc alloys supplied by Brock Alloys (GB), and havecast the centraliser body from these materials.

The sliders preferably engage in pre-formed slots or apertures in thebody, typically on the outer surface, so that they protrude slightlyfrom the aperture or slot to extend slightly proud of the outermostsurface of the centraliser body. The blades are especially good mountsfor the sliders, as hollows or slots etc can be readily machined or castinto the material of the or each blade.

The sliders can be of any desired shape but they typically provide abearing surface with a lower friction coefficient than the body of thecentraliser or (in some embodiments) the blades. This enhances thefriction coefficient of the centraliser and helps it to slide pastobstructions more easily.

The sliders are typically in the form of buttons, patches or strips thatare either attached to or inserted into the outer surface of the body,so that they will contact the wellbore or other surface in use beforethe rest of the body of the centraliser. However the sliders can incertain embodiments comprise the blades with a simple coating of lowfriction material thereon.

The sliders can be formed from low friction materials to reduce theforce needed to slide the centraliser past or along a surface orprotrusion, and preferred low friction materials include engineeringplastics such as polymeric ethylene compounds, nylon compounds, or anylow friction plastics material. Particularly suitable compounds includePTFE, polyetheretherketone, carbon reinforced polyetheretherketone,polyphthalamide, polyvinylidene fluoride, polyphenylylene sulphide,polyetherimide, polyethylene, polysulphone, polyethersulphone,polybutyleneterephthalate, polyetherketoneketone, polyamides, phenolicresins or compounds, thermosetting plastics, thermoplastic elastomers,thermoplastic compounds or thermoplastic polyester resins, PETP, KetronPeek, Torlon, Nylatron, Ultrawear, and Fluorosint, and their chemicalequivalents and related compounds. Preferred coatings includemetal/plastic composites such as nickel/phosphorous embedded with PTFEor another low-friction substance.

The blades are preferably equidistantly distributed around the body fromone another. They preferably each extend circumferentially at leastpart-way around the body between longitudinally opposite ends to providea circumferential distribution of each of the well bore-contactingsurfaces. Each blade preferably has a radially inner root integral withthe body, and each blade's root is preferably circumferentially widerthan its radially outer edge.

The blades are preferably circumferentially wider at one end (typicallythe lower end) of the centraliser than at the other (typically lower)end in use. The centraliser preferably has four to six blades.

Longitudinally opposite ends of the blades and/or the body may bechamfered or tapered so as to facilitate passage of the centraliser downa well bore.

Preferably the assembly also includes a centraliser stop collar forlongitudinally restraining a casing centraliser when installed on atubular casing, the stop collar comprising a ring having a substantiallycylindrical bore extending longitudinally therethrough, the bore beingdimensioned to fit around the casing, and the ring having longitudinallock means for longitudinally locking the collar to the casing.

The lock means preferably comprises one or more internally threadedbores extending radially through the ring, and a screw-threaded fastenerin each internally threaded bore. Each fastener can typically be screwedinto contact with the casing to lock the collar in place.

The ring may be formed of any suitable material such as metals likesteel, but some embodiments are formed from a zinc alloy which ispreferably the same alloy as that from which the centraliser is formed.Each internally threaded bore may be defined by an initially separatethread insert forming an integral part of the collar when fabricated,for example by being cast into the ring, and the thread inserts may beformed of materials which are substantially different from that of thering, e.g. of brass or steel as compared to a zinc alloy.

Preferably, the centraliser is rotatable on the casing.

The or each centraliser may be longitudinally restrained by a respectivestop collar installed upon casing at or adjacent one end of therespective centraliser. One or more centralisers may be longitudinallyrestrained by a respective pair of stop collars, one of the pair of stopcollars being installed on said casing at or adjacent eachlongitudinally opposite end of the respective centraliser.

The inner surface of the centraliser may have a low friction coating orslider. In some embodiments of the invention the centraliser is coatedon its inner and outer surfaces (or on selected parts of these surfaces)with PTFE-impregnated nickel using Niflor™ materials available fromSurface Technology plc, preferably using the electroless process knownin the art for coating articles with such materials. By slider we meanany member that can present a surface against which the wellbore canbear when the centraliser is in use. The slider can be a button, blockor other 3-dimensional object embedded in or adhered to the body orblade, or can be a strip or coating that has negligible or even variabledepth. The provision of sliders on the body or blade can be especiallybeneficial as the sliders can be concentrated on the outermost areas ofthe body or blade which will have the most contact with the wellboreinner surface, and can therefore be renewed or replaced easily. Indeed,since some areas of the centraliser outer surface can encounter moreabrasive conditions than others (e.g. the shoulders of the blades) thesecan be provided with sliders that are specifically shaped to present thelow friction surface of the slider over the whole of the area sufferinghigh abrasion, without having to over-engineer the whole of the body orblade. Also, the sliders on e.g. the shoulders can be made thicker thanthe sliders provided on less abraded areas of the body or blades e.g. inthe middle of the blades, so that the low friction surfaces on the highabrasion areas do not wear out before those on less abraded regions ofthe centraliser. Therefore, all of the low friction surfaces of thecentraliser need not be of the same depth, or shape.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Examples of a centraliser in accordance with the invention will now bedescribed with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view from above and to one side of a firstexample of a centraliser;

FIG. 2 is a plan view from above of the first example;

FIG. 3 is an underneath view of the first example;

FIGS. 4 and 5 are respectively radial (plan) and circumferential (side)views of a blade forming part of the first example;

FIGS. 6, 7 and 8 are respectively plan, perspective and side views of acasing stop collar suitable for use in conjunction with the centraliserof FIG. 1;

FIG. 9 is a perspective view of a combination of stop collars and acentraliser;

FIG. 10 is a perspective view of a third example of a centraliser; and

FIG. 11 is a perspective view of a fourth example of a centraliser.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIGS. 1-3, a casing centraliser 10 has a generallycylindrical body 12, and an array of five blades 14 integrally formedwith the body 12 and spaced around it at equal intervals. A cylindricalbore 16 extends longitudinally through the centre of the body 12, thebore 16 having a substantially uniform diameter dimensioned to be aclearance fit around the wellbore casing (not shown in FIGS. 1-8 ). Eachof the blades 14 (see also FIGS. 4 & 5) not only extends betweenlongitudinally opposite ends of the body 12, but also extendscircumferentially part-way around the periphery of the centraliser 10.The skewing of the blades 14 ensures that their respective radiallyouter edges 18 collectively provide a circumferentially substantiallyuniform well bore-contacting surface for the centraliser 10, as mostparticularly shown in FIGS. 2 and 3.

Each of the blades 14 has a respective radially inner root 20 integralwith the body 12. In each of the blades 14, the root 20 has a greatercircumferential width than the outer edge 18, i.e. the cross-section ofeach blade 14 tapers towards the well bore-contacting periphery of thecentraliser 10. The individual and collective shapes of the blades 14,and of the longitudinal fluid flow passages defined between adjacentpairs of the blades 14, gives the centraliser 10 improved flowcharacteristics and minimises the build-up of trapped solids during useof the centraliser 10.

Longitudinally opposite ends of the blades 14, and of the body 12, arechamfered to assist in movement of the centraliser 10 up/down a wellbore.

Although the blades 14 are shown separately from the body 12 in FIGS. 4and 5 (and while the blades 4 could be separately formed andsubsequently attached to the body 12 by any suitable means) it ispreferred that the centraliser body 12 is fabricated as a one-piecearticle, preferably by being precision cast in a suitable metal oralloy.

The blades 14 in the first embodiment have strips 19 ofpolytetrafluoroethylene (PTFE) attached to their outer surfaces 18 tobear against the inner surface of the well bore. The PTFE strips areglued or otherwise attached to the blades. No modification is necessaryfor the blades to receive the strips 19, but strip attachment plates(not shown) can be provided on the outer surfaces 18 if desired toimprove the ability of the strip 19 to attach to the particular metaletc of the body 12. The strips 19 preferably extend from one end of theblades 14 to the other and follow the contours of the blades 14 at theends where they bend into the body 12. However, this is not necessaryand the strips could alternatively be applied in patches along theblades 14. The strips 19 can be applied to each of the blades 14, but asatisfactory embodiment could equally carry the strips 19 (or patches)on one or a few blades 14.

Further PTFE strips 5 are similarly attached to the inner surface ofbody 12.

The strips 5, 19 are of PTFE, but could alternatively be formed fromother low-friction material such as those mentioned above or frompolyetheretherketone, carbon reinforced polyetheretherketone,polyphthalamide, polyvinylidene fluoride, polyphenylylene suiphide,polyetherimide, polyethylene, polysuiphone, polyethersulphone,polybutyleneterephthalate, polyetherketoneketone, polyaxnides, phenolicresins or compounds, thermosetting plastics, thermoplastic elastomers,thermoplastic compounds or thermoplastic polyester resins.

Since the bore 16 is a clearance fit around the casing and since thebore 16 lacks any means of tightly gripping a normally dimensionedcasing, the centraliser 10 can not only rotate freely around the casingbut also move freely along the casing (unless and until the centralisercollides with an obstruction, for example a protruding casing joint). Astop collar 50 as illustrated in FIGS. 6, 7 and 8 can optionally be usedto restrain the centraliser 10 substantially at its preferred locationalong the casing without impairing relative rotation of centraliser andcasing.

The stop collar 50 comprises an undivided ring 52 having a bore 54 aboutequal in diameter to the bore 16 in order to fit alongside thecentraliser 10 on the same casing. The ring 52 is radially penetrated byfive internally threaded holes 56. The ring 52 is cast of the same zincalloy as the centraliser 10, and five thread inserts 58 are either castinto the ring 52 to form the threaded holes 56, or subsequently screwedinto or pressed into a previously cast ring.

In use, the ring 52 is fitted around the casing to restrain thecentraliser in the desired location. A grub screw 60 is then screweddown each of the threaded holes 56 to tighten against the underlyingcasing (not shown in FIGS. 6-8) so as to lock the collar 50 onto thecasing.

The locked-on collar 50 then provides an abutment which stopslongitudinal movement of the centraliser in one direction while allowingfree relative rotation of the centraliser and the casing. While a singlestop collar would normally be located under a centraliser on vertical ornear-vertical casing to prevent unrestricted dropping of the centraliserdown the casing, circumstances may dictate that a stop collar be locatedabove a centraliser, or that a respective stop collar be used at eachend of a centraliser, for example in deviated wells.

FIG. 9 shows a modified form of casing centraliser 100, fitted aroundhollow tubular casing 102 which is located within a well bore 104. Themodified centraliser 100 is essentially the same as the centraliser 10described above, and differs principally in the dimensions andproportions of its blades 106, and in that the blades 106 are formedseparately of low friction material such as PTFE or another as indicatedabove, and are later attached to the body of the cast metal centraliser100.

The blades 106 are circumferentially wider at the lower end of thecentraliser 100 than they are at the upper end. FIG. 9 also illustratesthe manner in which the centraliser will hold casing out of directcontact with the well bore and centrally within the well bore, inpreparation for subsequent cementing.

In a modification to the FIG. 9 embodiment which is identical inappearance, the blades are cast separately from any suitable materialsuch as zinc alloy, and are then coated with a low-friction coating suchas the Niflor™ material referred to above, and preferably using theelectroless process also referred to above. The treated blades are thenattached to the body of the centraliser by any suitable means such asfixings or adhesives etc.

In the case of casing located within larger diameter casing,centralisers can be employed on the inner casing to hold it out ofdirect contact with the outer casing.

FIG. 10 shows a further embodiment of a centraliser 110 with a body 112and blades 114 with radially outward surfaces 118. The centraliser body112 is typically of cast metal such as Zinc or Aluminium etc, and theblades 114 have apertures 115 to receive cylindrical slider blocks 119of PTFE or a similar low friction material. The slider blocks 119 engagein the apertures 115 and can be held there by adhesive, fixings or byany other convenient means. The slider blocks 119 protrude by 2-5 mmfrom the surface of the blades 114 so as to contact the wellbore surfaceand reduce the friction as the centraliser engages it.

FIG. 11 shows a further embodiment of a centraliser 120 with a body 122,blades 124 having radially outward surfaces 128 and slots 125 along thelength of each blade to receive an elongate slider 129 of PTFE or asimilar low-friction material as described above. The sliders 129 engagein the slots 128 in the same way as the blocks 119 engage in theapertures 115, and can be held there by adhesive, fixings or simply bytheir own shape which can be selected to be slightly oversized to retainthe slider in the slot or other aperture as required, thereby obviatingthe requirement for any additional form of fixing. The sliders 129protrude above the surface 128 of the blades 124 by 2-5 mm to bearagainst the well bore surface and reduce the friction involved in movingthe centraliser against the well bore (or other) surface.

The slider can be selected from various different shapes such as arcuateor polygonal blocks, e.g. squares, triangles, ovals, circles, stripsetc.

Modifications and improvements can be incorporated without departingfrom the scope of the invention.

What is claimed is:
 1. A casing centraliser for use in a wellbore havinga body with a bore therethrough for receiving a tubular to becentralised, the body having at least one low friction slider on theouter surface thereof, wherein the at least one slider provides abearing surface with a lower coefficient of friction than the rest ofthe casing centraliser, wherein the at least one slider comprises any ofa button, a patch, a strip, a block, or a blade that is coupled to theouter surface of the centraliser such that the at least one slidercontacts the wellbore surface prior to the centraliser contacting thewellbore surface.
 2. A casing centraliser as claimed in claim 1, whereinthe at least one slider bears against the interior surface of a bore inwhich the casing centraliser is placed and wherein the at least oneslider comprises at least a part of a blade.
 3. A casing centralisor asclaimed in claim 2, having more than one blade, and wherein the bladesare arranged in a peripheral array circumferentially distributed aroundthe body to define a flow path between each circumferentially adjacentpair of blades.
 4. A casing centraliser as claimed in claim 2, whereinthe radially outer edge of at least one blade has at least one lowfriction slider in a form selected from the group consisting ofcoatings, strips and blocks, applied thereto.
 5. A casing centraliser asclaimed in claim 2, wherein the at least one blade is coated with a lowfriction material.
 6. A casing centraliser as claimed in claim 1,wherein the inner surface of the casing centraliser also has at leastone low friction slider in a form selected from the group consisting ofblocks, coatings and strips, applied thereto.
 7. A casing centreliser asclaimed in claim 1, wherein the body of the casing centraliser comprisesat least one of the group consisting of Zinc, Aluminum, Steel and acomposite material.
 8. A casing centraliser as claimed in claim 1,wherein the at least one slider engages in an aperture in the body, sothat the slider protrudes slightly from the aperture to extend slightlyproud of the outermost surface of the casing centraliser body.
 9. Acasing centraliser as claimed in claim 1, having more than one slider,wherein at least some of the sliders are of a form selected from thegroup consisting of buttons, patches and strips that are coupled to theouter surface of the body, so that in use they will contact the interiorsurface of a bore in which the casing centraliser is placed before therest of the body of the casing centraliser.
 10. A casing centraliser asclaimed in claim 1, wherein at least a portion of the at least oneslider is formed from compounds selected from the group consisting of:engineering plastics; polymeric ethylene compounds; nylon compounds;Polytetrafluoroethylene (PTFE); polyetheretherketone; carbon reinforcedpolyetheretherketone; polyphthalamide; polyvinylidene fluoride;polyphenylylene sulphide; polyetherimide; polyethylene; polysulphone;polyethersulphone; polybutyleneterephthalate; polyetherketoneketone;polyamides; phenolic resins or compounds; thermosetting plastics;thermoplastic elastomers; thermoplastic compounds; thermoplasticpolyester resins; Polyethlene Terephthalate (PETP); and chemicalequivalents and related compounds.
 11. A casing centraliser as claimedin claim 1, wherein the at least one slider is not, of even depth.
 12. Acasing centraliser as claimed in claim 1, having more than one sliderand wherein at least one slider differs from the others.
 13. A casingcentraliser as claimed in claim 1, wherein the body has a low frictioncoating and the inner and outer surfaces of the body are at leastpartially coated with one of the group consisting of PTFE-impregnatednickel and phosphorous.
 14. A casing centraliser as claimed in claim 1,wherein the body has a low friction coating of uneven depth.
 15. Acasing centraliser as claimed in claim 1, having at least one blade, andwherein the at least one slider engages in an aperture in the at leastone blade, so that the slider provides slightly from the aperture toextend slightly proud of the outermost surface of the blade.
 16. Acasing centraliser as claimed in claim 1, having at least one blade andhaving more than one slider, wherein at least some of the sliders are ofa form selected from the group consisting of buttons, patches and stripsthat are coupled to the outer surface of the at least one blade, so thatin use they will contact the interior surface of a bore in which thecasing centraliser is placed before the rest of the casing centraliser.17. A casing centraliser for use in a wellbore having opposing surfaces,the centraliser having a body with a bore therethrough for receiving atubular to be centralised; the body having at least one blade; the bladehaving at least one low friction slider provided its outer surface; andwherein the at least one slider provides a bearing surface with a lowercoefficient of friction than the rest of the centraliser; wherein theblade is coupled to the outer surface of the centralizer such that theslider contacts the wellbore surface prior to the centraliser contactingthe wellbore surface.